swss test changes for Virtual chassis

sonic-net/sonic-buildimage#4709 is a PR for create/delete a virtual chassis.
Similar to create_vnet.sh, virtual_chassis.py is used to create the virtual chassis test
environment.
The changes here are to
1. Add a test_virtual_chassis.py to verify the virtual chassis
2. support virtual chassis topology in conftest.py to run the test_virtual_chassis.py.
   Notable changes in conftest.py are
   * Unlike create_vnet, virtual_chassis does not use a sw container.
   * New DockerVirtualChassisTopology class to setup the connections when the
   vitual chassis containers restart.

tests/conftest.py

@@ -389,7 +389,6 @@ def vct_connect(self):
         for intf in intfs:
             self.ctn.exec_run( "ip link set dev " + intf + " up" )
         if "inband_address" in out:
-            iaddr = out[ "inband_address" ]
             ifpair = out[ "inband_intf_pair" ]
             ifname = out[ "inband_intf" ]
             self.vct.connect( ifname, ifpair, pid )